P-870HN-5
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b Support Notes
All contents copyright (c) 2009 ZyXEL Communications Corporation.
NAT Port forwarding
NAT/Multi-NAT Introduction
What is Multi-NAT?
The NAT (Network Address Translation-NAT RFC 1631) is the translation of an
Internet Protocol address used within one network to a different IP address known
within another network. One network is designated as the
inside
network and the
other is the
outside
. Typically, one company maps its local inside network addresses
to one or more global outside IP addresses and "unmaps" the global IP addresses on
the incoming packets back into local IP addresses. The IP addresses for NAT can be
either fixed or dynamically assigned by the ISP. In addition, you can designate servers,
e.g., a Web server and a Telnet server, on your local network and make them
accessible to the outside world. If you do not define any servers, the NAT offers the
additional benefit of firewall protection. In such case, all incoming connections to
your network will be filtered out by the CPE, thus preventing intruders from probing
your network.
For more information on the IP address translation, please refer to RFC 1631,
The IP
Network Address Translator (NAT)
.
How NAT works?
If we define the local IP addresses as the Internal Local Addresses (ILA) and the
global IP addresses as the Inside Global Address (IGA), see the following figure. The
term 'inside' refers to the set of networks that are subject to translation. The NAT
operates by mapping the ILA to the IGA required for communication with hosts on
other networks. It replaces the original IP source address (and TCP or UDP source
port numbers) and then forwards each packet to the Internet ISP, thus making them
appear as if they came from the NAT system itself (e.g., the CPE router). The CPE
keeps track of the original addresses and port numbers, so the incoming reply
packets can have their original values restored.
